1. Field of the Invention
The present invention relates to a technique for a structure of a boom constituting a working machine in an excavation machine typified by a power shovel or the like.
2. Background Art
A power shovel is an excavation machine well known as a hydraulic shovel excavation machine. The power shovel is basically structured to include a self-propelled lower traveling body and an upper rotational body rotatable by 360 degrees on the lower traveling body. The upper rotational body includes a boom and an arm, and an attachment such as a bucket is attached to the boom and the arm. Generally, in relation to the power shovel, the boom, the arm, and the bucket are generically referred to as “operating part” and the boom and the arm are generally referred to as “front”.
The boom is a cylindrical structure including three support point parts of an arm support point part, a boom support point part, and a boom cylinder rod support point part. To create a space in which the attachment can rotate during an excavation operation, the boom is foamed into a “dogleg” shape in a side view. An arm cylinder actuating the arm is arranged above the boom.
To keep balance while the power shovel operates and to resist a load during the excavation operation, it is considered that the boom needs to have a strength and to be reduced in weight. Conventionally, a boom configured to bond left and right side plates to upper and lower plates by welding and to have a rectangular cross section has been most popular. In such a boom, a cross-sectional area of a central portion that requires strength is made large. A boom having a triangular cross section and a boom having a generally trapezoidal cross section (for example, Patent Document 1) are also well known.    Patent Document 1: Japanese Patent No. 3165483